With the career goal of becoming an independent investigator, Dr. Uriel Felsen describes a mentored research project and specific career development plan which will prepare him to study the implementation of HIV treatment and prevention strategies for underserved populations. Despite a growing number of biomedical, behavioral, and structural interventions shown to be efficacious in preventing HIV transmission, important questions remain as to how to optimally implement such interventions in order to translate these advances into real-world outcomes, particularly among underserved populations that remain at greatest risk. HIV testing is a critical component of HIV treatment and prevention, and strategies for HIV testing in the Emergency Department (ED) setting can potentially impact the evolution of the domestic epidemic. This proposal aims to: 1) Identify barriers to and facilitators of implementing an expanded HIV testing strategy in the ED; 2) Use these findings to adapt an existing ED-based HIV testing strategy into a new strategy; 3) Test the process- and prevention-related outcomes of this new strategy; and 4) Develop a generalizable Implementation Science tool for implementing and evaluating an ED-based HIV testing strategy. To accomplish these aims, Dr. Felsen will pursue training in Implementation Science, qualitative methods, development of health services interventions, and the design and analysis of quasi-experimental studies. With completion of these activities, along with intensive mentorship, Dr. Felsen will be prepared to disseminate and test his tool for implementing and evaluating an ED-based HIV testing strategy across multiples sites for his career goal of becoming an independent investigator.